user: implement mlibc as the libc, finally.

It's finally done..

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/mlibc/tests/posix/ffs.c

@@ -0,0 +1,18 @@
+#include <assert.h>
+#include <strings.h>
+#include <limits.h>
+
+int main(void){
+    assert(ffs(0x8000) == 16);
+    assert(ffs(0) == 0);
+	assert(ffs(INT_MAX - 1) ==  2);
+	assert(ffs(INT_MAX) == 1);
+	assert(ffs(INT_MIN) == (int)(sizeof(int) * CHAR_BIT));
+	assert(ffs(INT_MIN + 1) == 1);
+
+	for (int i = 1; i < 0x1000; i++) {
+		assert(ffs(i) - 1 == __builtin_ctz(i));
+	}
+
+    return 0;
+}